Enhanced or unregulated production of tumor necrosis factor α (TNF-α) has been implicated in inflammatory, allergic, and autoimmune diseases. It has been shown that adenosine 3′,5′-cyclic monophosphate (cAMP) plays a role in TNF-α production. Elevation of cAMP in inflammatory leukocytes inhibits their activation and the subsequent release of inflammatory mediators, including TNF-α. The primary cellular mechanism for the inactivation of cAMP is the breakdown of cAMP by a family of isoenzymes referred to as cyclic nucleotide phosphodiesterases (PDE). The inhibition of PDE, in particular type IV PDE (PDE4), is effective in the inhibition of TNF-α release.
For example, 2-[1-(3-ethoxy-4-methoxyphenyl)-2-methylsulfonylethyl]-4-acetylaminoisoindoline-1,3-dione is a PDE4 inhibitor that is currently under investigation as an anti-inflammatory for the treatment of a variety of conditions, including asthma, chronic obstructive pulmonary disease, psoriasis and other allergic, autoimmune and rheumatologic conditions. S-enantiomer form of 2-[1-(3-ethoxy-4-methoxyphenyl)-2-methylsulfonylethyl]-4-acetylaminoisoindoline-1,3-dione can be prepared by reacting (S)-aminosulfone 1 with intermediate 2.

Existing methods for synthesizing (S)-aminosulfone 1 involve resolution of the corresponding racemic aminosulfone by techniques known in the art. Examples include the formation and crystallization of chiral salts, and the use of chiral high performance liquid chromatography. See, e.g., Jacques, J., et al., Enantiomers, Racemates and Resolutions (Wiley Interscience, New York, 1981); Wilen, S. H., et al., Tetrahedron 33:2725 (1977); Eliel, E. L., Stereochemistry of Carbon Compounds (McGraw Hill, N.Y., 1962); and Wilen, S. H., Tables of Resolving Agents and Optical Resolutions p. 268 (E. L. Eliel, Ed., Univ. of Notre Dame Press, Notre Dame, In., 1972). In one example, as depicted in Scheme 1 below, (S)-aminosulfone 1 is prepared by resolution of racemic aminosulfone 3 with N—Ac-L-Leu. Racemic aminosulfone 3 is prepared by converting 3-ethoxy-4-methoxybenzonitrile 4 to enamine intermediate 5 followed by enamine reduction and borate hydrolysis. This process has been reported in U.S. Patent Application Publication No. 2010/0168475.

The procedure for preparing an enantiomerically enriched or enantiomerically pure aminosulfone, such as compound 1, may be inefficient because it involves the resolution of racemic aminosulfone 3. Thus, a need exists as to asymmetric synthetic processes for the preparation of an enantiomerically enriched or enantiomerically pure aminosulfone, particularly for manufacturing scale production. Direct catalytic asymmetric hydrogenation of a suitable enamine or ketone intermediate is of particular interest because it eliminates the need for either classic resolution or the use of stoichiometric amount of chiral auxiliary, and thus, may be synthetically efficient and economical.